Transonic flow is where air flows above, at, and below the speed of sound at the same time at different points on an object. For example, the air on a wing flows faster, so that air could be supersonic while the air flowing over the body of an airplane could be subsonic. There is a loss of control effectiveness when an airplane flies in the transonic range. The transonic range is the range where the Mach number varies from 0.8 to 1.2. And, in this range, there is a rapid increase in drag, which reduces the airspeed and aircraft losses control. In fluid dynamics, the Mach number is a dimensionless quantity representing the ratio of flow velocity past a boundary to the local speed of sound. When an aircraft approaches the speed of sound, the airflow over the wing reaches supersonic speed before the airplane itself does, and a shock wave forms on the wing. The airflow behind the shock wave breaks up into a turbulent wake, increasing drag. If so, it's because as the airplane approaches the speed of sound, the air traveling over the forward part of the top of the wing exceeds the speed of sound, creating a shockwave that causes the center of pressure to move rearward with increasing speed.